1. Field of the Invention
The invention relates to luminescent screens for flat image display devices and somewhat more particularly to such screens wherein the individual luminescent dots are separated from one another by a black border layer.
2. Prior Art
Flat image display devices functioning in accordance with the principles of gas discharge displays, as plasma panels or plasma displays are known, for example see German Offenlegungsschrift No. 24 12 869 (generally corresponding to U.S. Pat. No. 3,956,667). In such flat image display devices, i.e., image display devices using a flat picture screen, a fine-grained luminescent screen with a high light efficiency is required. Plasma in such devices functions as the actual cathode, from which an electron beam is drawn over a perforated control panel having a matrix control for each luminescent point to be excited. In comparison to a classic cathode beam image tube, where a single electron beam must reach all luminescent points, which, with typical large image tube dimensions, requires very high beam acceleration, the electron beam in gas discharge display devices is a relatively low energy beam. Further, due to the flat screen structure and the gaseous atmosphere in such devices, high acceleration voltages are not possible. Therefore, it is extremely important for luminescent screens in flat image display devices to obtain a high light efficiency after conversion of electron energy into light or after light generation from the luminescent points or phosphor dots via, for example, impingement on such points or dots by ultraviolet radiation.
For improving light efficiency in classic cathode ray tubes it is known to coat the luminescent material points or dots on their inner surfaces (i.e., away from the viewing surface) with a mirroring ring or reflecting metal layer, for example as described in U.S. Pat. No. 3,858,083. In this manner, light emitted by excited luminescent material toward the back or rear of a so-called luminescent dot is largely reflected by this metal layer and added to the light emitted toward the front. However, the excited electrodes are also weakened by this metal layer. Such partial electron de-energization is not overly detrimental for high-energy cathode beam electrons but with low-energy electrons used in flat plasma display devices, the use of such metal mirror is detrimental.
It is also known in classic cathode ray tubes to isolate individual luminescent points or dots from each other by opaque borders, for example from the earlier referenced U.S. Pat. No. 3,858,083. In conjunction with flat image display devices, such opaque borders are described and claimed in co-pending U.S. application Ser. No. 012,348 filed Feb. 15, 1979, now U.S. Pat. No. 4,243,735.